In case it helps someone else who lands here like I did, after a bit of researching I found out that using ISO8601 format also works:

git log --since="2014-02-12T16:36:00-07:00"
This will give you precision down to the second. Note: you can also use:

git log --after="2014-02-12T16:36:00-07:00"
git log --before="2014-02-12T16:36:00-07:00"
git log --since="1 month ago"
git log --since="2 weeks 3 days 2 hours 30 minutes 59 seconds ago"